The grid voltage controls the current flow thru the tube.  Usually, the tube needs to be turned off by a fairly high negative voltage on the grid.  Zero volts, ground or no connection, will typically turn the tube to max on.

To SET the bias you adjust the negative voltage on the grid when there is no input audio signal.  The actual voltage depends on many things, including tube type, tube age, and how well the tube is working.  To MEASURE the bias you measure how much current is flowing through the cathode (or plate) of the output tube(s).  Typically, there is a known value resistor put in the cathode circuit that you can measure the voltage across.  This can give you the cathode current by applying Ohms law, but typically the directions will have already done that for you and will say something like "adjust trimpot R422 until the voltage at tp7 is 2.3vdc".  

Some amps, self bias, are designed so there is no need for a bias adjustment.  This is convenient, but you pay for it in that you are never able to set the amp up to give you all the power its capable of giving.


Dan  

Setting the bias, whether it is self bias (not user adjustable) or fixed bias (user adjustable), puts the amplifiers operating point right in the center of the usable operating region.  From there, an input signal can swing both positive and negative a maximum amount before running into physical limits of the amp that cause clipping and distortion.  So, properly set bias allows the amp to put out its maximum power before it starts to clip.  If the bias is not set right, the amp will clip early, before it reaches maximum power.  Also, if the bias is wrong, the output waveform will clip on the top or bottom first, whereas a properly biased amp will clip symmetrically on both top and bottom as the amp is asked for more power than it can supply.

Dan

I throw the shades down--sound is better.

Knuckleheads!?

While I could be accused of that, there is no way that Lon could ever be considered of being one.

This reeks of unconscious bias.

I know this because I had to sit through a 4 hour class about bias at work ( you can't let anyone cry anymore). I'm now an expert on it.